As densities and complexities of integrated circuits (ICs) continue to increase, inspecting photomask patterns become progressively more challenging. Every new generation of ICs has denser and more complex patterns that currently reach and exceed optical limitations of lithographic systems. To overcome these optical limitations, various Resolution Enhancement Techniques (RET), such as Optical Proximity Correction (OPC), have been introduced. For example, OPC helps to overcome some diffraction limitations by modifying photomask patterns such that the resulting printed patterns correspond to the original desired patterns. Such modifications can include perturbations to sizes and edges of main IC features, i.e., printable features. Other modifications involve additions of serifs to pattern corners and/or providing nearby sub-resolution assist features (SRAFs), which are not expected to result in printed features and, therefore, are referred to as non-printable features. These non-printable features are expected to cancel pattern perturbations that would otherwise have occurred during the printing process. However, OPC makes mask patterns even more complex and usually very dissimilar to resulting wafer images. Furthermore, OPC defects often do not translate into printable defects.
Non-printable and printable features have different effects on resulting printed patterns and often need to be inspected using different inspection parameters, e.g., sensitivity levels. Areas containing non-printable features are typically “de-sensed” to avoid false positives during inspection. Conventional inspection methods generally rely on user defined characteristics, such as feature sizes, for differentiating between printable and non-printable features. This approach is sometimes referred to as rule-based, where a user defines specific rules for feature identification. However, in many modern ICs non-printable features often can be larger than printable features. Furthermore, identifying areas containing non-printable is complex and consumes much inspection resources.